This invention relates to LixMn2O4-zFz of a spinel structure for use as a cathode active material in a lithium secondary cell and a method for producing LixMn2O4-zFz, which lithium secondary cell uses an intercalation compound such as metallic lithium or lithium-graphite (lithium-carbon) as an anode active material.
The lithium secondary cell has been acknowledged to provide safety as well as a high energy density and high cyclability, and is now prevalent in a cellar phone market at an increasing rate. In addition, the same secondary cell has recently been promoted to come into practical service in the field of large-scaled batteries for use on electric vehicles and for use in distributed storage systems. In the past, the lithium secondary cell has typically employed LiCoO2 as a cathode material. LiCoO2 is able to absorbingly store and release lithium.
The above cobalt is costly and limited in resource amount, and would drain off in absolute amount when a demand for batteries continues to increase at a current speed and further when the large-scaled batteries are virtually brought into practical service. Therefore, earnest studies have been made all over the world to replace such a costly cathode material with manganese that is rich in resource and harmless to the environment. However, a lithium-manganese oxide characterized by high voltage and a high energy density has a drawback to charge-discharge cycle characteristics and, in particular, a short cycle life due to dissolution of manganese into electrolyte solution in a hot atmosphere.
In view of the above, an object of the present invention is to provide a lithium-manganese oxide for use in a lithium secondary cell cathode and a method for producing the lithium-manganese oxide, designed to allow a cathode active material to exhibit improved charge-discharge cycle characteristics.
The above object is achievable by the use of a lithium-manganese oxide for use in a lithium secondary cell cathode, having a spinel structure expressed by a chemical compositional formula of LixMn2O4-zFz (1.12xe2x89xa6Xxe2x89xa61.20, 0 less than Z less than 0.16) and having a lattice constant ranging from 8.220 to 8.230 xc3x85.
The method for producing the lithium-manganese oxide comprises the steps of: mixing electrolytic or chemically synthesized manganese dioxide, lithium salt, and fluoride together so as to reach a Li/Mn mole ratio of 0.56xe2x89xa6Li/Mnxe2x89xa60.60 and a F/Mn mole ratio of 0 less than F/Mn less than 0.08; and thereafter subjecting the mixture to primary heat treatment at a temperature ranging from 400 to 600xc2x0 C. and then to secondary heat treatment at a temperature falling within the range of 700 to 800xc2x0 C. The lithium salt can be lithium nitrate, lithium hydroxide, or lithium carbonate.